As a power generator that converts thermal energy into electrical energy, a thermionic power generator that generates electromotive force using thermion emission is known. For example, JP-A-2009-238690 (corresponding to US 2011/0050080 A1) discloses an electron emission element in which a first diamond layer and a second diamond layer are formed on a conductive substrate. In the electron emission element, phosphorus is used as a dopant doped to the first diamond layer, and nitrogen is used as a dopant doped to the second diamond layer so as to increase thermionic current.
However, the thermionic current of the electron emission element is not enough to be used for a thermionic power generator, and a power generation efficiency is low.